... In addition, foreign broker-dealers that, from outside of the United States, induce or attempt to induce securities transactions by any person in the United States, or that use the means or instrumentalities of interstate commerce of the United States for this purpose, also must register. This includes the use of the internet to offer securities, solicit securities transactions, or advertise investment services to U.S. persons. See Securities Exchange Act Release No. 39779 (March 23, 1998)
http://www.sec.gov/rules/interp/33-7516.htm.
How hard is it to add "Service not available to US citizens" to your conditions?
I'm not a company or even an independent broker, I don't have to saw anything, plus I don't know every jurisdiction and treaty in the world, I may not be able to do business with people in Iran, North Korea, or Burkina Faso, and I don't care, I'm not starting a business here!